Polymer electrolyte fuel cells (or solid polymer fuel cells) have a basic component called a membrane electrode assembly (MEA). The MEA is obtainable by laminating an electrode membrane (a catalyst layer or an electrode catalyst membrane) on either side of a polymer electrolyte membrane and putting the resulting laminate between a fuel gas supply layer and an air supply layer; the electrode membrane contains, as a main component, a platinum group metal catalyst supported on a carbon powder, the polymer electrolyte membrane is an ion exchange membrane, and the fuel gas supply layer and the air supply layer are an electroconductive porous membrane. For the MEA, both electrolyte membrane and electrode membrane contain an ion exchange polymer. The electrolyte membrane and the electrode membrane are usually formed by a casting method and/or a coating method. A method of laminating the electrolyte membrane and the electrode membrane usually comprises allowing an electrolyte membrane formed on a support to closely contact with an electrode membrane formed on a support, thermocompression bonding these membranes at about 130 to 150° C. (about 150 to 200° C. according to a material used) and a pressure of about 1 to 10 MPa, and then separating the supports from these membranes. Thus, as the support, a release film is used. The release film requires a moderate (or reasonable) peelability from the electrolyte membrane and the electrode membrane and a moderate (or reasonable) adhesion to the electrolyte membrane and the electrode membrane. The ion exchange polymer contained in the electrolyte membrane and the electrode membrane has a unique structure that has a main chain being a highly releasable fluoropolymer and a side chain containing a poorly releasable sulfonic acid group. This makes it difficult to predict the peelability behavior and to balance the peelability and the adhesion. The release film for the production of a fuel cell further requires heat resistance for reasons of production process. The release film, which is produced by a roll-to-roll processing in terms of efficient production, also requires flexibility. As the release film, a fluorine-containing film is widely used. The fluorine-containing film has excellent heat resistance, releasability, and stain resistance, while the film is expensive and less combustible in waste incineration after use, and easily generates a poisonous gas. The film, which has a low elastic modulus, is difficult to produce by a roll-to-roll processing. In place of the fluorine-containing film, a release film containing a cyclic olefin polymer is also reported.
Japanese Patent Application Laid-Open Publication No. 2010-234570 (JP-2010-234570A, Patent Document 1) discloses a release film made of a cycloolefin copolymer and also describes a release film formed by coating a solution of a cycloolefin copolymer on a substrate film such as a poly(ethylene terephthalate) film. This document discloses that the cycloolefin copolymer has a glass transition temperature of usually not lower than 50° C. and most preferably not lower than 160° C. and that a common cycloolefin copolymer has an upper limit of Tg of about 250° C. In working examples of the document, a solution containing a copolymer of ethylene and norbornene is cast on a poly(ethylene terephthalate) film using a flow-casting apparatus to form a release film having a thickness of 0.5 μm.
Unfortunately, this release film, which has a low heat resistance, a peelability insufficient for a high-temperature production process, and an insufficient toughness, easily cracks or fractures when being produced by a roll-to-roll processing.
Japanese Patent No. 5017222 (JP-5017222B, Patent Document 2) describes a copolymer for forming a polarizing film for a liquid crystal display; the copolymer comprises (A) 10 to 69% by mol of a structural unit derived from propylene, (B) 1 to 50% by mol of a structural unit derived from a C6-12α-olefin, and (C) 30 to 89% by mol of a structural unit derived from norbornene, and has a weight-average molecular weight of 50,000 to 1,000,000. This document discloses that the copolymer has a glass transition temperature of 50 to 250° C. (particularly, 80 to 200° C.). In working examples of the document, a copolymer having a glass transition temperature of 92 to 168° C. is prepared.
The document discloses neither a fuel cell nor a release film. If the film is used as a release film for producing a fuel cell, the film would be unsuitable due to an insufficient heat resistance thereof.
Japanese Patent Application Laid-Open Publication 2009-298999 (JP-2009-298999A, Patent Document 3) describes an optical film of a cyclic olefin addition copolymer obtainable from a cyclic olefin (A) monomer unit and an α-olefin (B) monomer unit having 5 or more carbon atoms; the proportion of the structural unit (A) and that of the structural unit (B) are 80 to 99% by mol and 1 to 20% by mol, respectively, in 100% by mol of the total amount of the structural units (A) and (B). This document discloses that the cyclic olefin addition copolymer has an excellent heat resistance, a high transparency, a low water absorbency, an excellent moldability, an excellent toughness, and a low coefficient of linear expansion and describes that the copolymer is suitable for an optical substrate for a display, or other applications. The document discloses that the cyclic olefin addition copolymer has a glass transition temperature of 200 to 400° C. (particularly, 245 to 300° C.). In working examples of the document, a copolymer having a glass transition temperature of 245 to 262° C. is prepared.
This document also fails to describe a fuel cell. The document discloses a release film just as an example of electric insulating members. The document has no description about releasability as a purpose or an advantage or about releasability evaluation. If the film is used as a release film for producing a fuel cell, the film would be unsuitable for an application that requires a high heat resistance.